HELLO I HAVE A 1969 20HP JOHNSON SEAHORSE JUST WONDERING CAN I BEEF UP TO 25HP? MODEL #20R-69D. AND IF SO WHAT CARB WOULD I NEED. AND WHAT OTHERS WOULD I NEED. ANY HELP WOULD BE GREAT. THANKS.
This topic and question will not go away. ---Takes more than a carburetor change.-----There were some internal changes. to make more power.----And I believe crankshaft was balanced to allow it to rev at 6000 RPM.-----The 18 / 20 HP was rated at 4500 RPM.-----The 25 was rated at 5500 RPM.
18-20-25hp motors had different combinations of pistons, connecting rods, crankshaft, and cylinder heads as well as different carbs. And no, you can't just slap a 25hp head on an 18 or 20. It's all or nothing. OMC saw you coming on this one and made sure it wasn't an easy do.
Reeds had some differences too.
This is one case/example where the are no differences in the Block/Ports, but that doesn't mean the Port Timing is the same between the engines